1. Field of the Invention
The present invention relates to a switching power converter for suppressing common mode noise.
2. Description of the Related Art
Currently, switching mode power supply must meet EMI international standard such that filters become necessities. In general, conductive EMI noise comprises differential mode noise and common mode noise. The current-loop of the differential mode noise is formed between Live wire (L) and Neutral (N) wire, and its value is related to the magnitude of ripple current of input of the power supply. The current-loop of the common mode noise is formed between power supply Live wire (L)/Neutral (N) wire and ground, and which is produced by high current (i=C*dv/dt) of discharging and charging on parasitic capacitors. In filter design, the differential mode noise can be suppressed by differential mode inductor and X capacitor, and the common mode noise can be suppressed by common mode inductor and Y capacitor.
Nowadays, though the common mode inductor and Y cap can be used to reduce the common mode noise, the size of them is still a big problem in the high requirement of power density of power supply. Accordingly, it is needed to reduce the original noise of the power supply to meet the requirement. Moreover, J.P. patent No. 05-153782 entitled “Noise Preventive Device” issued on Jun. 18, 1993 to NOMURA TOSHIHIRO teaches a reduced noise power converter which is provided by adding a noise preventive circuit for canceling a noise current flowing to a parasitic capacitance between a power converter circuit and a metal case. In addition, U.S. Pat. No. 5,724,236 entitled “Power converter transformer having an auxiliary winding and electrostatic shield to suppress noise” issued on Mar. 3, 1998 to Oglesbee and John W. teaches a reduced noise power converter.
Currently, some of the methods for reducing common mode noise of the power supply comprise the following: 1. Adapting symmetrical circuit structure, such as full bridge converter, push-pull converter. Voltage jump points of such circuits have the properties of the same magnitude, reverse-complement phase and hence common mode noise is smaller. 2. Utilizing two asymmetrical circuits such as two half-bridge converter for executing reverse-complement operating. During operation, when one converter creates positive voltage jump to charge on parasitic capacitors, another converter creates negative voltage jump to discharge on the parasitic capacitors, if charging current and discharging current are substantially the same, then common mode current passing through ground is almost zero and hence common mode noise detected on LISN is smaller. 3. Suppressing common mode noise using reverse-voltage principle between driving voltage and switching voltage. In general, the driving voltage is smaller than the switching voltage and hence capacitance of an external capacitor is adjusted to acquire common mode current of the same magnitude and reverse phase. 4. Adapting for compensation method, i.e. making a reverse-complement phase voltage on an original converter structure, utilizing common mode current produced by the above voltage balancing the common mode current of the original converter to get a smaller total common mode noise. The reverse-complement phase voltage can be produced by applying an external transformer or amplifier, or produced by applying an additional winding on main transformer. 5. Adding Faraday shield between the transformer's windings to reduce parasitic capacitance of primary side relative to secondary side of the transformer to acquire a smaller common mode noise.
Although all the aforementioned methods can reduce common mode noise produced by a voltage jump of primary side of the switching mode power supply, they do not mention common mode noise produced by another voltage jump of secondary side. Basing on our research, voltage jump of secondary side also affects common mode noise, especially when using normal Faraday shield, voltage jump of the secondary side may create a larger common mode noise than that produced by voltage jump of the primary side.
Therefore, in view of the above-mentioned drawbacks of prior art, a new switching power converter can be provided to achieve a smaller common mode noise.